1. Field of the Invention
This invention relates to a method and apparatus or system for recovering methane from methane hydrates. In one aspect, this invention relates to a method and apparatus for recovering methane from subterranean methane hydrate deposits. In another aspect, this invention relates to a method and apparatus for in situ recovery of methane from subterranean methane hydrate deposits.
2. Description of Related Art
Methane hydrate, also known as methane clathrate, is a cage-like lattice of ice inside which are trapped molecules of methane, the primary component of natural gas. If methane hydrate is either warmed or depressurized, it reverts back to methane and water. Methane hydrate fields or deposits generally occur under the Arctic permafrost or beneath the ocean floor. While global estimates vary considerably, the energy content of methane occurring in hydrate form is immense, possibly exceeding the combined energy content of all other known fossil fuels. However, future production volumes are speculative because methane production from hydrate has not been documented beyond small-scale field experiments. Thus, with the known reserves of oil and non-hydrate gas diminishing on a daily basis, the need for a viable method for large-scale removal and recovery of this heretofore untapped reserve of methane is substantial.
There are at present three approaches for the in-situ release and recovery of methane from a methane hydrate deposit or field. The first of these involves heating the methane hydrate, which requires only a small percentage of the heating value of the trapped methane. Under this approach, a heated fluid is pumped down to the subterranean methane hydrate deposit. It has been found, however, that, because of heat losses incurred during transmission of the heated fluid to the methane hydrate deposit, the amount of energy required to supply the required heat into the methane hydrate deposit nearly equals the heating value of the released methane. In-situ combustion could reduce such heat losses, but is said to be difficult to establish in a hydrate deposit and would result in undesirably high hydrate deposit temperatures.
The second approach involves reducing the in-situ pressure to a value below the dissociation pressure of the methane hydrate deposit. However, the energy required for methane dissociation must still be provided to the hydrate deposit. As a consequence, the methane hydrate deposit temperature decreases, thereby requiring even lower dissociation pressures or heating of the hydrate deposit. Thus, with this approach, it is necessary to mine the solid methane hydrates and pump a slurry to the surface. To date, no such mining system has been demonstrated to be economically feasible.
The third approach involves pumping carbon dioxide downhole to displace methane from the methane hydrates by formation of carbon dioxide hydrates. However, at the temperatures of the methane hydrate deposit, the reaction is prohibitively slow. In addition, under the conditions of a stable hydrate bed, methane hydrate reforms from available methane and water. Thus, to minimize the reformation of methane hydrate, it is necessary to heat the hydrate deposit.
U.S. Pat. No. 6,973,968 B2 to Pfefferle teaches a method and system for dissociating methane hydrate deposits in-situ in which an oxidizer fluid and a fuel supply, both at a pressure higher than that of the methane hydrate deposit, are supplied and delivered to the methane hydrate deposit and the fuel is combusted downhole using the oxidizer fluid to provide combustion products, which are placed in contact with a diluent fluid to produce a heated product fluid. The heated product fluid is injected into the methane hydrate deposit whereby methane is dissociated from the methane hydrate and made available for extraction. In accordance with one embodiment, carbon dioxide is provided to the methane hydrate deposit to promote the formation of carbon dioxide hydrates from the liberated methane hydrate water. Disadvantageously, all of the fluids, i.e. fuel, oxidizer, and CO2, required for implementation of this method are transported from sources above ground to the methane hydrate deposit, requiring substantial expenditures of energy and reducing the net heating value of the extracted methane.